1. Field of the Invention
The present invention relates to a method of grinding a back-side surface of a wafer such as a semiconductor wafer so as to thin the wafer.
2. Description of the Related Art
In a semiconductor device manufacturing process in which a face-side surface of a wafer composed of a semiconductor such as silicon is provided with a multiplicity of devices and the wafer is divided to obtain the devices as individual chips, a step of grinding the back-side surface of the wafer so as to thin the wafer is conducted at the stage of wafer. The thinning of the wafer corresponds to thinning of device packages, and the wafer is thinned, for example, from an initial thickness of around 700 μm to about 200 μm. In accordance with the marked thinning nowadays, the wafers may in some cases be thinned to a very small thickness of 50 μm or 30 μm.
For grinding the back-side surface of a wafer, infeed grinding is generally used in which the wafer is suction held onto a vacuum chuck type chuck table, and, while revolving the wafer about its center by rotating the chuck table, a grinding tool such as a grindstone is pressed against the back-side surface of the wafer. In the case of grinding the back-side surface of a wafer in this manner, the face-side surface of the wafer is covered with a protective member so that the face-side surface does not make direct contact with the holding surface of the chuck table, in order to prevent electronic circuits of the devices from being damaged (refer to Japanese Patent Laid-open No. 2005-057052).
Examples of the protective member include a protective tape obtained by applying an acrylic pressure sensitive adhesive material to one side of a base sheet made of polyolefin or the like. In such a protective tape, in many cases, at least one of the base sheet and the pressure sensitive adhesive material has unevenness, if slight, in the thickness thereof. If such a protective tape is adhered to the face-side surface of the wafer and the back-side surface of the wafer is ground, the unevenness of thickness of the protective tape would be transferred to the wafer, resulting in unevenness of thickness of the wafer as a single body. If the finished thickness of the wafer is comparatively large and the unevenness of thickness of the protective tape is as tiny as about 3 μm, for example, the ratio of the thickness unevenness to the wafer thickness after grinding is so small as not to matter considerably. If the finished thickness of the wafer is as extremely small as about 30 μm, however, the thickness unevenness of 3 μm is as large as 10% of the wafer thickness and, therefore, it is difficult to make uniform the wafer thickness with high precision.
The above-mentioned patent document discloses a protective film formed by applying a fluid resist to the back-side surface of a wafer by a spin coating process or a printing process. However, even such a protective film has unevenness of thickness, so that the protective film is not superior to the protective tape from the viewpoint of thickness unevenness.